Luminescent rare-earth doped alkaline-earth fluorides have long been known, and have been employed for numerous purposes such as scintillation detectors and laser materials. CaF2 doped with such rare-earth species as Eu+3, Er+3, Tb+3 are well-known compositions. It is well-known that a rare-earth doped alkaline earth fluoride will exhibit luminescence when exposed to ultraviolet light.
Each rare-earth element when incorporated into an alkaline earth host lattice such as CaF2 exhibits a characteristic excitation spectrum; see, for example, FIG. 1 (101), and a characteristic emission or luminescence spectrum that depends upon the excitation wavelength employed; see, for example, FIG. 1 (102). The excitation spectrum is determined by monitoring the luminescence intensity at one wavelength while the specimen is illuminated over a range of wavelengths. The luminescence spectrum is determined by illuminating the specimen at a single wavelength corresponding to a peak in the excitation spectrum and determining the luminescence spectrum by scanning a detector over a range of wavelengths.
As shown in the figures, each such spectrum consists of a plurality of peaks at different wavelengths of light. The wavelengths at which the peaks occur are characteristic of each rare-earth element. No two rare-earth elements exhibit the same excitation or emission spectra; that is, the peaks in their spectra do not in general arise at the same wavelengths. To obtain luminescence, the rare-earth element must be excited by a light source that emits light at a wavelength corresponding to the location of one of the peaks in the excitation spectrum thereof. In general, the peaks in any one spectrum of rare-earth elements differ from one another in height or intensity, these differences in intensity being characteristic of the rare-earth element under particular conditions of measurement. These and related matters are all well-documented in the art. See for example, Martin et al., Atomic Energy Levels—the Rare-Earth Elements, U.S. Department of Commerce, National Bureau of Standards (1978).
Haubold et al., U.S. Published Patent Application 2003/0032192 discloses the use of doped luminescent inorganic compounds for marking goods, such as in use as so-called anti-theft or anti-counterfeiting security markers. Haubold et al., WO 03/052025 discloses printing but no details are provided. Rare-earth doped alkaline earth compositions are not disclosed in Haubold.
Considerable effort in the art is directed towards the provision of compositions useful as so called “security markers” or anti-counterfeiting marks on goods. The idea is to place an identifying mark on a manufactured article which will attest to its authenticity in the face of rampant piracy on a global scale. The mark is ideally invisible until inquiry is made using a particular wavelength of ultra-violet light which stimulates the rare-earth doped fluoride to luminesce with a characteristic spectrum.